Soliton pulse transmission over waveguide fiber lengths

ABSTRACT

The invention proposes to increase the signal/noise ratio in a long-haul transmission system by: 
     filtering noise ( 1 ) outside the range of wavelength of the signals transmitted, 
     shifting ( 2 ) the wavelength of the signals transmitted, and 
     filtering the signals transmitted ( 5 ) that have undergone wavelength shifting. 
     The wavelength of the signals can be shifted by widening the spectrum of the signals or by optical phase conjugation.

BACKGROUND OF THE INVENTION

The present invention relates to fiber optic transmission systems andmore particularly to optical links and networks having a very highcapacity and covering very long distances. By optical links with a veryhigh capacity is meant transmission systems providing a bit rate greaterthan 10 Gbit/s. By transmission systems covering very long distances ismeant systems covering propagation distances of the order of 5000 km ormore.

At present obtaining very high capacities on long systems is notpossible, apart from N*2.5 Gbit/s wavelength division multiplex systems,even though papers have been published reporting laboratory results thatare as yet incompatible with the requirements of real systems.

At present, RZ (return to zero) pulse transmission and NRZ (no return tozero) pulse transmission are widely used in long-haul fiber optictransmission systems. One problem for this type of system is that thesignal/noise ratio increases as the number of repeaters in the systemincreases, due in particular to amplified spontaneous emission (ASE)noise.

In the case of soliton signal transmission systems, to reduce theamplified spontaneous emission noise, and thereby to increase thesignal/noise ratio, it has been proposed, for example in EP-A-0 576 208,to use sliding guiding filter systems. This solution is based on theparticular nature of solitons and their capacity for selfregeneration.In other words, soliton signals track the sliding of the filters,whereas the amplified spontaneous emission noise is filtered out.

This solution also applies to types of signals other than solitonsignals. However, because it relies on self-phase-modulation of thesignals, it is difficult to implement for wavelength division multiplextransmission systems, because of crossed phase modulation betweenchannels. The passage of the signals through the sliding filters impliesa high level of self-phase-modulation, which goes hand in hand with ahigh level of crossed phase modulation. To obtain results with this typeof solution in a wavelength division multiplex transmission system itwould be necessary to separate the various channels well beyond the bandof a few nanometers available for the signals.

The transmission of signals in optical systems is also limited bynon-linear effects, such as the Kerr effect, the Brillouin effect, theRaman effect and four-wave mixing. G. P. Agrawal, “Nonlinear FiberOptics”, Academic Press, 1980 describes these non-linear effects. Theydepend on the level of noise in the optical fibers of the transmissionsystem.

SUMMARY OF THE INVENTION

The invention proposes a solution to the problem of increasing noise,and in particular of amplified spontaneous emission noise in a fiberoptic transmission system. It significantly improves the quality factorof transmission systems, especially systems of very high capacity andcovering long distances. The invention eliminates most of the noise atthe wavelengths of the signals transmitted and makes possible “linear”transmission that is not limited by the noise level; it also makespossible nonlinear transmission limited by the effects of the noise.Some embodiments of the invention reduce the timing jitter of thesignals.

To be more precise, the invention proposes a noise-reducing device for afiber optic transmission system, said device including

first means for filtering noise outside the range of wavelengths of thesignals transmitted,

means for shifting the wavelength of the signals transmitted, and

second means for filtering the transmitted signals that have undergonewavelength shifting.

The device advantageously includes second wavelength shifting means forreturning the signals that have undergone the second filtration to theirinitial wavelength.

In one embodiment the wavelength shifting means include means forwidening the spectrum of the signals.

The wavelength shifting means preferably include optical phaseconjugation means.

In one embodiment the filtration means include a Bragg filter.

The invention also provides a fiber optic transmission system includingat least one such device.

The invention further provides a method of reducing noise in a fiberoptic transmission system, said method comprising the steps of

filtering noise outside the range of wavelength of the signalstransmitted,

shifting the wavelength of the signals transmitted, and

filtering the signals transmitted that have undergone wavelengthshifting.

In one embodiment the method further includes a second wavelengthshifting step for returning the signals that have undergone the secondfiltration step to their initial wavelength.

The wavelength shifting step advantageously includes widening thespectrum of the signal.

The wavelength shifting step can also include conjugation of the phaseof the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the following description of embodiments of the invention, givenby way of example and with reference to the accompanying drawings, inwhich:

FIG. 1 shows the spectrum of signals in a transmission system;

FIG. 2 shows the spectrum of the signals after a first step offiltration in accordance with the invention;

FIG. 3 shows the spectrum of the signals after a step of wavelengthshifting in accordance with the invention;

FIG. 4 shows the spectrum of the signals after a second step offiltration in accordance with the invention;

FIG. 5 is a diagram of a noise-reducing device in accordance with theinvention;

FIG. 6 shows the spectra of the signals for wavelength shifting byoptical phase conjugation;

FIG. 7 is a graph of the power of signals received in a conventionaltransmission system; and

FIG. 8 is a graph of the power of signals received in a transmissionsystem using the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention proposes to filter the signals transmitted and then tosubject them to “non-linear hopping” so as to recover them in the rangeof wavelengths previously filtered, in which the noise has been reduced.It proposes various ways to effect the non-linear hopping, in otherwords to obtain a wavelength shift for the signals transmitted.

FIGS. 1 to 4 show the spectra of the signals in a first embodiment ofthe invention. In this embodiment, simple widening of the signalstransmitted is used to subject the signals to non-linear hopping. FIG. 1shows the spectrum of the signals. The noise level is of the order of N₀and the signal level is of the order of E. The signals have wavelengthsaround λ_(c), which is typically 1550 nm, the range of wavelengths inthe transmission system extending from 1530 to 1580 nm.

The invention proposes firstly to filter noise outside the range ofwavelengths of the signals. In the FIG. 2 example, the noise is filteredaround the wavelength λ₁, which is higher than the wavelength λ_(c). Aband-pass filter or a high-pass filter can be used to filter the noise,for example. For RZ pulses centered at 1550 nm, which typically have aspectral width of 0.2 nm, the noise can be filtered around a wavelengthλ₁ that is about 1 nm above the center wavelength of the pulses. Thewidth of the filter can be of the order of 0.3 nm; it is advantageouslychosen so that it does not reduce the power of the signals transmittedby more than 0.5 dB.

The invention then proposes to subject the signals transmitted tonon-linear hopping, in other words to subject them to a wavelengthshift, which does not apply much or at all to the noise. In the FIG. 3example, simple widening of the signal is used to subject the signals tonon-linear hopping. It can be obtained by disposing at the output of theamplifier a section of fiber having zero chromatic dispersion for awavelength close to that of the signals transmitted, for example. Theeffect of such a fiber section is to explode the spectrum intosub-components, as a function of the power and the wavelength. Thechoice of the amplification gain can be optimized to encourage thewidening of the signals transmitted at λ_(c) to maximize the proportionof the signals transmitted around the wavelength λ₁.

FIG. 3 shows the shape of the widened signal. Note that this solution ispreferably applied to RZ pulses. It has the advantage that it also peaklimits the pulses, in other words reduces the excess power of thepulses.

The invention then proposes to filter the signals and the noise outsidethe range of wavelengths centered on λ₁. Such filtering recovers theoriginal part of the signals transmitted whilst eliminating noise at theoutput. In fact, in the first filtration step of FIG. 2, the noise iseliminated around λ₁. In the second filtration step, only signals aroundλ₁ are recovered, i.e. signals in a range of wavelengths in which thenoise is low. In fact, the non-linear character of the widening ensuresthat the noise does not pass much or at all into the range around thewavelength λ₁. FIG. 4 shows the shape of the signals obtained afterfiltration around λ₁. The level of the signals around λ₁ is denoted E′in the figure. The value of E′ is of the same order as the value E ofthe power of the signal; the loss by shifting and filtering the signalsis compensated in this embodiment by amplifying the signals used forwidening the signals.

In this embodiment, with values of the order of 1 nm for the differencebetween the wavelengths λ₁ and λ_(c), Bragg filters operating inreflection can be used to filter the signals, or filters known in theart having analogous performance.

FIG. 5 is a diagrammatic representation of a noise-reducing device inaccordance with the invention. The device has first filter means 1 thatfilter noise around the wavelength λ₁ with no or little power reductionin the range of wavelengths of the signals transmitted. The device thenhas wavelength shifting means 2; in the embodiment shown in the figure,the wavelength shifting means include an amplifier 3 followed by a loopof fiber having zero chromatic dispersion at the wavelength λ_(c) of thesignals transmitted. As explained above, the shifting means widen thespectrum of the signals transmitted so that the signals extend into therange of wavelengths in which noise has previously been filtered. Afterthese shifting means the device includes second filter means 5 thatfilter signals outside the range of wavelengths around λ₁. Signals witha slightly different wavelength (a higher wavelength in the example) arerecovered at the output of the device, with a higher signal/noise ratio.

To achieve greater shifts, up to a few nanometers, other signal wideningmeans can be used. In the second embodiment of the invention, describednext, optical phase conjugation is used to shift the signalstransmitted. FIG. 6 shows the shape of the signals. It shows in fullline the spectrum of the signal after filtering noise around thewavelength λ₁. The spectrum of a pump is shown in dashed line. The thickline shows the spectrum of the signal obtained around the wavelength λ₁by four-wave mixing of the transmitted signals and the injected pump.

The second embodiment has the advantage of achieving greater differencesbetween the wavelength λ_(c) and λ₁, in other words a greater wavelengthshift. This guarantees that the system has greater acceptance, inparticular in the face of variations in the wavelength of the sourcesending the signal. Also, in the case of wavelength division multiplextransmission, the solution of the second embodiment shifts all thechannels as a block, for example by means of a pump with a wavelengthgreater than the greatest wavelength of the channels of the multiplex.The second embodiment applies not only to RZ pulses but also to NRZpulses. Finally, compared to the first embodiment, the second embodimentdoes not induce any frequency conversion of the signal, whose spectrumis modified less; this facilitates repetition of filtering andwavelength shifting.

A third embodiment of the invention proposes to use wavelengthconverters to apply the wavelength shift to the transmitted signals. Inaccordance with the invention, any wavelength converter device known inthe art can be used, not only a converter based on optical conjugationof the signals in four-wave mixing, as in the second embodiment.

The three embodiments of the invention produce transmitted signals witha lower noise level around a wavelength shifted relative to the initialwavelength of the signals. The improvement in the signal/noise ratio conbe of the order of 10 dB.

FIG. 7 shows the shape of the signals received at the receiver of aconventional fiber optic transmission system. Time in picoseconds isplotted on the abscissa axis and power in milliwatts is plotted on theordinate axis. The signals are RZ pulses at a bit rate of 10 Gbit/s, andpropagate in 5000 km of optical fiber with an injected power of −4 dBm.The figure shows a high noise level; the figure of merit of the link,measured in a manner known in the art, is of the order of 5.8.

FIG. 8 shows a similar graph for a link in accordance with the inventionin which noise-reducing means of the type described above are providedin the receiver. As shown in the figure, the noise level is clearlylower, and the figure of merit is of the order of 18.4.

In accordance with the invention, a higher power can be injected intothe link than is possible in the prior art.

The invention has been described with reference to FIGS. 1 to 8 in itssimplest application. It is possible to perform transformations of thetype described with reference to FIGS. 1 to 4 several times on a link.Thus, after recovering the signal around the wavelength λ₁ at the outputof FIG. 4, it can be returned to the wavelength λ_(c). For this, theprocess is the exact reverse of that described with reference to FIGS. 1to 4: the first step is to filter noise around the wavelength λ_(c), ifthis has not been done during the second filtration step of FIG. 4; thewavelength of the signals is then shifted from λ₁ to λ_(c) by a processthat is the reverse of that of FIG. 3. The signals around the wavelengthk are then filtered. This “reversal”, in terms of wavelength, furtherreduces the noise level; it has the advantage of returning the signalsto the initial range of wavelengths.

The steps described above can also be repeated several times on thelink, increasing or decreasing the wavelength. The number of repetitionswithout changing the direction of variation of the wavelength dependsonly on the transmission range of the fibers, amplifiers and othercomponents of the transmission system.

The position of the noise-reducing device or devices in accordance withthe invention depends on the required effects and the embodiment used.Noise can be produced in accordance with the invention anywhere alongthe transmission system. The noise is preferably reduced sufficientlyearly on for the pulses to have a much higher level than the noise. Asignal/noise ratio of the order of 5 dB/nm or more is suitable.

In FIGS. 7 and 8, the noise is reduced in accordance with the inventionin the receiver. It could have been reduced sooner; however, thisembodiment is particularly advantageous for repeaterless systems. Forsystems using repeaters a noise-reducing device in accordance with theinvention can be inserted after or before a repeater.

In the case of a wavelength division multiplex transmission system,noise can be reduced in all the channels of the multiplex at the sametime, as in the second embodiment. If is also possible to operatechannel by channel, after demultiplexing the signal, and then toremultiplex the signal afterwards. This solution has the advantage thatthe respective positions of two channels can be interchanged to preservethe spectral shape of the channels of the multiplex.

A noise-reducing device in accordance with the invention isadvantageously associated with active devices such as phase or amplitudemodulators for reducing timing jitter. In this way not only the noisebut also the timing jitter of the signal are reduced.

Of course, the present invention is not limited to the examples andembodiments described and shown, and lends itself to many variants thatwill be evident to the skilled person. It applies in particular to allfiber optic transmission systems, regardless of the signals transmitted(RZ or NRZ pulses, solitons or other signals), on a single channel orwith wavelength division multiplexing. It applies equally well torepeaterless transmission systems (with no electrically active elementsin the link) and transmission systems using repeaters.

What is claimed is:
 1. A noise-reducing device for a fiber optictransmission system, said device including: first means for filteringnoise outside the range of the signals transmitted, means for shiftingthe wavelength of the signals transmitted, and second means forfiltering the transmitted signals that have undergone wavelengthshifting.
 2. The device according to claim 1, further including a secondwavelength shifting means for returning the signals that have undergonesecond filtration to their initial wavelength.
 3. The device accordingto claim 1, wherein the wavelength shifting means include means forwidening the spectrum of the signals.
 4. The device according to claim1, wherein the wavelength shifting means include optical phaseconjugation means.
 5. The device according to claim 1, wherein thefiltration means include a Bragg filter.
 6. A fiber optic transmissionsystem including a noise reducing device, said noise reducing devicecomprising: a first mechanism configured to filter noise outside therange of the signals transmitted, a shifting mechanism configured toshift the wavelength of the signals transmitted, and a second mechanismconfigured to filter the transmitted signals that have undergonewavelength shifting.
 7. A method of reducing noise in a fiber optictransmission system, said method comprising: filtering noise outside therange of wavelength of the signals transmitted, shifting the wavelengthof the signal transmitted, and filtering the signals transmitted thathave undergone wavelength shifting.
 8. The method according to claim 7,said method further including a second wavelength shifting step forreturning the signals that have undergone the second filtration step totheir initial wavelength.
 9. The method according to claim 7, whereinthe wavelength shifting step includes widening the spectrum of thesignal.
 10. The method according to claim 7, wherein the wavelengthshifting step includes conjugation of the phase of the signal.
 11. Anoise-reducing device for a fiber optic transmission system, said deviceincluding: a first mechanism configured to filter noise outside therange of the signals transmitted, a shifting mechanism configured toshift the wavelength of the signals transmitted, and a second mechanismconfigured to filter the transmitted signals that have undergonewavelength shifting.
 12. The device according to claim 11, furtherincluding a second wavelength shifting mechanism configured to returnthe signals that have undergone filtration at said second mechanism totheir initial wavelength.
 13. The device according to claim 11, whereinthe wavelength shifting mechanism include a widening means configured towiden the spectrum of the signals.
 14. The device according to claim 11,wherein the wavelength shifting mechanism includes an optical phaseconjugation mechanism.
 15. The device according to claim 11, wherein thefirst or second filtration mechanism includes a Bragg filter.